It doesn't really make sense

I get 400+ Mb to modem. I have 100% signal strength to my PS4, but only get between 1-15 Mb. I know there are limitations to wifi, but that large of a loss with 100% signal strength? I'm sure this community is sick of wifi issues, but it just confounds me. Maybe there's something I can do with the playstation? I tried forwarding ports the other day, no difference.